Frequency hop pilot technique for a control system that reduces distortion produced by electrical circuits

ABSTRACT

A control system comprising an electrical circuit that produces distortion where the electrical circuit has a frequency band of operation. A pilot modulated carrier signal is frequency hopped about the frequency band of operation. The hopping pilot modulated carrier signal does not interfere with any signal applied to or generated by the electrical circuit as it is not at any time located within the frequency band of operation. Information obtained from the hopping pilot modulated carrier is provided to the control system which uses such information to cancel the distortion produced by the electrical circuit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to a control system comprisingan electrical circuit that uses a pilot signal to substantially canceldistortion produced by the electrical circuit and more particularly to atechnique of frequency hopping about the frequency band of operation ofthe electrical circuit.

[0003] 2. Description of the Related Art

[0004] Electrical signals when applied to electrical circuits are oftendistorted as a result of being processed by the circuits. Alsoelectrical circuits generate signals for various useful purposes. Thedistortion comprises any undesired signals produced by the electricalcircuits which are added to or are somehow combined with the applied orgenerated signals. A well known technique for substantially cancelingdistortion produced by an electrical circuit uses a control systemcoupled to the electrical circuit to which a pilot signal is applied.The applied pilot signal is detected by the control system. The appliedpilot signal can have a single spectral component (i.e., one frequency)of a certain amplitude or the applied pilot signal can comprise aplurality of spectral components of various amplitudes. Typically, theapplied pilot signal has an amplitude that is at least 60 dB lower thansignals applied to or generated by the electrical circuit. The controlsystem obtains information (about the distortion) from the detectedpilot signal and uses that information to substantially cancel thedistortion produced by the electrical circuit.

[0005] A control system comprises at least one circuit which usesexternal signals or signals generated by the at least one circuit toprocess signals applied to the at least one circuit. A particularimplementation of the technique discussed above is shown in FIG. 1.

[0006]FIG. 1 depicts a control system, comprising two feed forward loops(loop 1 and loop 2) and detection circuit 132. A pilot signal is appliedto electrical circuit 108 via coupler 105. Electrical circuit 108 can beany electrical and/or electronic (e.g., Radio Frequency (RF) linearamplifier, power amplifier) circuit. Loop 1 comprises coupler 105, Gain& Phase circuit 104, splitter 102 and delay circuit 126. Coupler 105 istypically a device which combines two or more input signals and allowsaccess to all or a portion of the combined signal. A coupler is alsoused to obtain a portion of a signal appearing at its input and output.Gain & Phase circuit 104 is typically a circuit which modifies theamplitude and phase of signals applied to its input based on the valuesof control signals applied to its control inputs (not shown). Splitter102 is a circuit with one input and at least two outputs where a signalapplied to the input is substantially replicated at the outputs. Delaycircuit 126 is typically a circuit which applies a certain amount ofdelay to a signal applied to its input.

[0007] When a signal is applied to the input of the control system(i.e., to splitter 102), the distortion experienced by the appliedsignal due to electrical circuit 108 is isolated at point A (i.e., path123). In particular, an input signal is applied to splitter 102.Splitter 102 substantially replicates the input signal on paths 103 and127. In path 103 the input signal is applied to Gain & Phase circuit104, coupler 105 and electrical circuit 108. In path 127, the inputsignal is delayed by delay circuit 126 and then fed to cancellationcircuit 124 via path 125. Although not shown, it will be readilyunderstood to those skilled in the art that the amplitude and phase ofthe input signal on path 125 can be detected (using well known detectioncircuitry) and converted to control signals that are applied to thecontrol inputs (not shown) of Gain and Phase circuit 104. Using coupler112, a portion of the input signal (plus any distortion produced byelectrical circuit 108) appearing at the output of electrical circuit108 is fed to cancellation circuit 124 via path 113. Cancellationcircuit 124 can be implemented as a combiner circuit which has at leasttwo inputs and one output. A combiner circuit combines signals appliedto its inputs and transfers the combined signal to its output. Gain andPhase circuit 104 is adjusted such that the amplitude and phase of theinput signal on path 113 are modified resulting in that signal beingsubstantially 180° out of phase (+/−1°) and relatively the sameamplitude (i.e., substantially the inverse) with the input signal onpath 125 such that when the two signals are combined by cancellationcircuit 124 they substantially cancel each other leaving the distortion(produced by electrical circuit 108) at point A (path 123). Thus Loop 1is designed to isolate the distortion produced by electrical circuit108.

[0008] Loop 2, which comprises delay circuit 114, coupler 116, Gain &Phase circuit 122, and amplifier 120, is designed to use informationobtained by Detection circuit 132 from a pilot signal applied toelectrical circuit 108 to substantially cancel the distortion producedby electrical circuit 108. In particular, a pilot signal is applied toelectrical circuit 108 via coupler 105. The pilot signal (processed byelectrical circuit 108) appears on path 115 and at the output of coupler116, i.e., on path 117. The pilot signal also appears at point A on path123 after having propagated through path 113 via coupler 112. A portionof the pilot signal processed by electrical circuit 108 is fed todetection circuit 132 via coupler 130 and path 128. Detection circuit132 comprises well known circuits (e.g., Log detector/amplifier, Sample& Hold circuit, Null circuit) to detect signal characteristics (e.g.,amplitude, spectral content, phase response) of the pilot signal. Someor all of the characteristics may have been altered due to thedistortion effects of electrical circuit 108. Detection circuit 132detects the characteristics of the input and uses this information togenerate control signals on path 131 to cause Gain & Phase circuit 122to modify the pilot signal. The pilot signal at point A is modified suchthat the pilot signal appearing on path 118 is substantially the inverse(relatively same amplitude, 180° out of phase, +/−1°) of the pilotsignal appearing on path 115. Amplifier 120 provides additional gain tothe output of Gain & Phase circuit 122. The additional gain iscalculated such that the signal appearing on path 118 has an amplitudesubstantially equal to the amplitude of the signal on path 115. Delaycircuit 114 is designed such that the two pilot signals arrive atcoupler 116 at substantially the same moment; that is, the two pilotsignals are substantially synchronized (aligned in time) to each other.When the two pilot signals are combined by coupler 116 they cancel eachother.

[0009] Detection circuit 132 now has the information that allows Gain &Phase circuit 122 to modify distortion appearing at point A and thuscancel distortion appearing at the output of electrical circuit 108.When an input signal is applied to the control system, any distortionproduced by electrical circuit 108 is isolated at point A (on path 123)as discussed above. The signal on path 115 is the input signal(processed by electrical circuit 108) plus any distortion produced byelectrical circuit 108. The distortion at point A is modified by Gainand Phase circuit 122 based on the information (i.e., signalcharacteristics) obtained from the previously applied pilot signal sothat the distortion on path 129 is substantially the inverse of thedistortion on path 115. The distortions on path 115 and path 118 arecombined at coupler 116 causing the distortions to substantially canceleach other resulting in an output signal that is substantially free ofdistortion.

[0010] Electrical circuit 108 has a bandwidth that defines a frequencyband of operation. It is desirable that the pilot signal be spectrallylocated substantially in the middle of the frequency band of operationof electrical circuit 108 because the distortion experienced by such apilot signal tends to be substantially similar to the distortionexperienced by a signal applied to or generated by electrical circuit108. However, placing the pilot signal anywhere in the band of operationof electrical circuit 108 causes interference to occur between the inputsignal and the pilot signal adding more distortion to the input signal.The interference is any interaction between the pilot signal and anapplied or generated signal that adversely affects one or morecharacteristic (e.g., amplitude, frequency, phase) of the applied orgenerated signal and/or the pilot signal. Thus, interference not onlydistorts any signal that is applied or generated by the electricalcircuit, but also affects the pilot signal. As discussed above, thepilot signal is typically {fraction (1/1000)} th the amplitude ofapplied or generated signals and thus would be interfered with by suchsignals. A distorted pilot signal provides inaccurate information aboutthe distortion and thus the very purpose of such a pilot signal isdefeated. Also, even when the pilot signal is located in the middle ofthe frequency band of operation, it does not experience the distortionslocated in other parts (e.g., lower band or upper band) of the frequencyband of operation. What is therefore needed is to use a pilot signalthat obtains information about the entire frequency band of operation ofthe electrical circuit and does so without interfering with any signalsapplied to or generated by the electrical circuit.

SUMMARY OF THE INVENTION

[0011] The present invention is a control system comprising anelectrical circuit that produces distortion where the electrical circuithas a frequency band of operation. A carrier signal modulated by a pilotsignal is applied to the electrical circuit and is frequency hoppedabout the frequency band of operation of the electrical circuit. Afterthe pilot modulated carrier signal has frequency hopped at least onceabout the frequency band of operation of the electrical circuitinformation about the distortion produced by the electrical circuit isobtained from the pilot signal. There is no interference between thefrequency hopped pilot modulated carrier signal and any signal appliedto or generated within the band of operation of the electrical circuitbecause the pilot modulated carrier signal is not hopped within thefrequency band of operation of the electrical circuit. The informationobtained is used by the control system to substantially cancel thedistortion produced by the electrical circuit.

[0012] The control system of the present invention also comprises afirst feed forward loop and a second feed forward loop coupled to theelectrical circuit. The control system of the present invention furthercomprises a Carrier circuit coupled to a Single Side Band (SSB)modulator which is coupled to the electrical circuit. The control systemof the present invention still further comprises a Detection circuitcoupled to the carrier circuit and the second feed forward loop. TheCarrier circuit is configured to generate a frequency hopped carriersignal. The SSB modulator is configured to generate a single side bandpilot signal and to modulate the carrier with the single side bandpilot. The Carrier circuit is further configured to cause the pilotmodulated carrier signal to frequency hop about the frequency band ofoperation of the electrical circuit and thus not interfere with anysignal applied to or generated within the frequency band of operation ofthe electrical circuit. After at least one hop, as the pilot modulatedcarrier signal hops about the frequency band of operation of theelectrical circuit, the Detection circuit obtains information about thedistortion produced by the electrical circuit and provides theinformation to the second feed forward loop. The first feed forward loopis configured to isolate the distortion produced by the electricalcircuit. The second feed forward loop is configured to use theinformation obtained by the Detection circuit to substantially cancelthe distortion produced by the electrical circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 depicts a control system comprising two feed forward loopsand detection circuitry;

[0014]FIG. 2 depicts a control system of the present invention;

[0015]FIG. 3 depicts the frequency response of the electrical circuit ofthe control system of the present invention showing a pilot modulatedcarrier signals at the hopping frequencies.

DETAILED DESCRIPTION

[0016] Referring to FIG. 2, there is shown a control system of thepresent invention comprising electrical circuit 108 coupled to a firstfeed forward loop (i.e., loop 1) and a second feed forward loop (i.e.,loop 2). The control system of the present invention further comprisesCarrier circuit 184 coupled to Single Side Band (SSB) modulator 182 andto Detection circuit 186. Electrical circuit 108 produces distortionwhich is isolated at point A by loop 1. Loop 2 uses information obtainedfrom Detection circuit 186 to cancel the distortion produced byelectrical circuit 108.

[0017] The information obtained by Detection circuit 186 is gleaned froma pilot modulated carrier signal applied to electrical circuit 108 (viapath 146 and coupler 105) with the use of carrier circuit 184 and SingleSide Band modulator 182. Carrier circuit 184 and SSB modulator 182 causethe pilot modulated carrier signal to frequency hop about the frequencyband of operation of electrical circuit 108. The frequency hopping ofthe pilot modulated carrier signal is the generation of a signal sothat, at different instants, the signal is spectrally located below andabove the frequency band of operation of electrical circuit 108; thepilot modulated carrier signal is not at any time located within thefrequency band of operation of the electrical circuit. Therefore, thereis no interference between the hopping pilot modulated carrier and anysignal applied to or generated by electrical circuit 108. After thepilot modulated carrier signal has frequency hopped at least once aboutthe frequency band of operation of electrical circuit 108, Detectioncircuit 186 obtains information from the pilot signal and provides thatinformation to Null circuit 132 via path 136. Null circuit 132 generatescontrol signals that cause Gain & Phase circuit 122 to modify thedistortion at point A such that the distortion is cancelled withdistortion on path 115 by coupler 116.

[0018] Referring to FIG. 3, there is shown a frequency response (300) ofelectrical circuit 108 with a spectral components 306 representing thepilot modulated carrier signal. The frequency response is a chart orgraph showing how a particular characteristic (e.g., amplitude, phase)of an electrical circuit corresponds with frequency. It should be notedthat the particular frequency response shown is for illustrativepurposes only and that electrical circuit 108 is not limited tofrequency response 300. The frequency band of operation is a range offrequencies within which electrical circuit 108 processes and/orgenerates signals. The boundaries of the frequency band of operation aredefined by lower frequency f_(L) and upper frequency f_(U). Frequenciesf_(L) and f_(U) correspond to the points (302, 304) at which thefrequency response is 3 dB below its maximum amplitude response (0 dB).Typically the bandwidth of a circuit is defined by the frequencies thatcorrespond to its 3 dB points. The frequency band of operation need notbe the bandwidth. One frequency hop is defined as when pilot modulatedcarrier signal 306 initially starts at a location (f₁) below thefrequency band of operation and then hops to location f₂ above thefrequency band of operation. The term frequency hopping is a well knownterm and is generally related to generating one or a plurality ofsignals at various frequency locations at different times. Depending onthe proximity of frequencies (f₁, f₂), i.e., the hopping frequencies, tofrequencies f_(L) and f_(U) the average of the information obtained fromthe hopping pilot modulated carrier signal reasonably reflects thedistortion within the band of operation.

[0019] Referring again to FIG. 2, Carrier circuit 184 comprisesFrequency Hop generator 166 coupled to Voltage Controlled Oscillator(VCO) 164 which is coupled to Splitter 162. Each of the outputs ofSplitter 162 is coupled to an amplifier (160, 170). One of the 5 outputsis applied to Mixer 172 of Detection circuit 186 and the other output isapplied to SSB modulator 182. Frequency Hop Generator 166 produces aswitching voltage signal. VCO 164 generates a carrier (i.e., a sinusoid)whose frequency is varied in direct correspondence to the switchingvoltage. As the voltage generated by Frequency Hop Generator 166switches from one value to another, the frequency of the carrier changesfrom one frequency to another. Thus, the Carrier circuit 184 generates acarrier signal that switches between a first frequency (f₁) and a secondfrequency (f₂).

[0020] SSB modulator 182 is a well known circuit shown here comprisingOscillator 148, 90° Phase circuit 150, Band pass Filters (BPF) 152, 153and balanced modulator 154. Oscillator 148 generates a pilot signal(e.g. a square wave) of a certain frequency that is applied to 90° Phasecircuit 150. 90° Phase circuit 150 divides the frequency of the squarewave and generates two square waves having that same desired frequencybut which are 90° out of phase with each other. Each of the signals isapplied to a Band pass filter (152, 153) having a pass band that allowcertain frequency components of the signals to pass through theircircuitry virtually unaffected. Band pass filters 152 and 153 are wellknown filter circuits that allow signals that fall within their passband to pass through virtually unaffected and filter out (orsignificantly attenuate) signals having frequencies that fall outside oftheir pass bands. It is well known that a square wave comprises manyfrequency components. The square wave generated by Oscillator 148 isconverted to a sinusoid by BPF 152 and 153 because many of the frequencycomponents of the square wave are filtered out by BPF 152 and 153. Thepass band is a frequency (or group of frequencies) about which the Bandpass filter frequency response is tuned. The outputs of the Band passfilters 152 and 153 are applied to balanced modulator 154. The output ofCarrier circuit 184 is also applied to balanced modulator 154 (via path158). Balanced modulator uses the pilot signal to modulate the carriersignal. Typically, when a carrier signal is modulated, other signalshaving frequencies commonly referred to as side band frequencies arecreated as a result of the modulation. The side band frequencies arefrequencies that are higher and lower than the frequency of the carrierby an equal amount. Because of the 90° phase relationship between thepilot signals applied to balanced modulator 154, one of the side bandsof the resulting pilot modulated carrier signal is effectivelysuppressed. The output of SSB modulator 182 is the carrier signalshifted in frequency by an amount equal to the frequency of the pilotsignal, and thus the output is a pilot modulated carrier signal.

[0021] The pilot modulated carrier signal is applied to electricalcircuit 108 and appears on path 117. A portion of the pilot modulatedcarrier signal is coupled to path 128 via coupler 130. The pilotmodulated carrier signal is applied to Band Stop Filter (BSF) 180. BSF180 is a filter circuit that rejects or significantly attenuates signalshaving certain frequencies and does not affect all other signals. BSF180 is designed to reject any signal falling within the band ofoperation of electrical circuit 108. The pilot modulated carrier signalis then applied to Mixer 172 via amplifier 168. An output of carriercircuit 184 is also applied to Mixer 172. Mixer 172 is a well knowncircuit having at least two inputs and at least one output where theoutput is the result of the two input signals being multiplied to eachother. It is well known that when two signals having certain frequenciesare mixed, the result are signals comprising frequencies which are thesum and difference of the frequencies of the two signals. The mixing oftwo signals having the same frequencies is known as synchronousdetection. Therefore, the pilot modulated carrier signal issynchronously detected by Mixer 172 and the output of Mixer 172 is fedto BPF 174 whose center frequency is equal to the frequency of the pilotsignal. The output of BPF 174 is therefore the pilot signal.

[0022] The pilot signal is applied to Log Detector 176 which detects acharacteristic (e.g., amplitude) of the pilot signal and converts saidcharacteristic to a voltage. The output of Log Detector 176 is appliedto Low Pass Filter (LPF) 178. LPF 180 acts as an averaging circuit andgenerates an average of a characteristic (e.g., amplitude, phase,frequency) of the detected pilot signal which contains information aboutthe distortion experienced by the pilot signal. The average is appliedto Null circuit 132 via path 136. Upon reception of a control signalfrom Frequency Hop Generator 166 (via path 134) Null circuit 132generates a control signal to Gain & Phase circuit 122 via path 131.Frequency Hop Generator 166 can send such a control signal after atleast one hop or after a plurality of hops about the frequency band ofelectrical circuit 108. Frequency Hop Generator 166 is configured tocontrol the rate at which the pilot modulated carrier hops about theband of operation of electrical circuit 108. Frequency Hop Generator isalso configured to control the length of time the pilot modulatedcarrier signal stays at a particular hopping frequency. The rate ofhopping and the length of time of each hop is such that CarrierDetection circuit 186 is able to provide information to Null circuit132. The control signals generated Null circuit 132 cause Gain & Phaseamplifier 122 to modify the distortion at point A such that it iscanceled with distortion appearing on path 115 with the use of coupler116.

I claim:
 1. A control system comprising an electrical circuit thatproduces distortion where the electrical circuit has a frequency band ofoperation and where a pilot signal is applied to the electrical circuitand the pilot signal is used by the control system to substantiallycancel the distortion, the control system further comprising: a Carriercircuit configured to generate a frequency hopping carrier signalmodulated by the pilot signal which pilot modulated carrier signal iscaused by the Carrier circuit to frequency hop at least once about thefrequency band of operation of the electrical circuit allowing thecontrol system to obtain information from the pilot modulated carriersignal that is used by the control system to substantially cancel thedistortion.
 2. The control system of claim 1 further comprising a SingleSide Band modulator configured to generate the pilot signal, the SingleSide Band modulator being coupled to the Carrier circuit uses the pilotsignal to modulate the frequency hopping carrier signal.
 3. The controlsystem of claim 1 further comprising a Detection circuit coupled to thecontrol system and configured to obtain information from the frequencyhopping pilot modulated carrier signal without any interference betweenany signal applied to or generated by the electrical circuit and thefrequency hopping modulated carrier signal.